The long range objective of this project is to understand the molecular and genetic basis of systems providing pathogenic bacteria the heritable capability to generate phenotypic diversity. diversity-generating systems provide pathogens an enormous potential for adapting to and withstanding their respective host environments. Since these systems often involve the surface structures on unicellular microbes, characterization of new molecular motifs of surface phenotypic variation is important in understanding microbial strategies for adaptation. It is proposed to characterize a newly-discovered microbial system of antigenic variation involving novel surface structures. This occurs in Mycoplasma hyorhinis, a pathogenic mycoplasma of swine. Mycoplasmas comprise a large group of specially-adapted wall-less procaryotic organisms that cause or are associated with several diseases in animals, including man. They appear to be associated with respiratory, genitourinary and other human disorders, now including acquired immunodeficiency syndrome. Mycoplasma hyorhinis has been shown to undergo high-frequency phase variation in the expression of variable lipoprotein (Vlp) surface antigens. structurally distinct Vlp products display non-coordinate phase variation and reversible size variation in vitro. Biochemical data and sequencing of recently identified vlp genes revealed a novel periodic structure in the C- terminal extracellular region of these otherwise highly conserved membrane- anchored lipoproteins. It is proposed to examine the structural and molecular genetic basis for Vlp antigenic and phase variation in order to understand how this unique molecular adaptation may provide these specialized microbes with the capacity to generate extensive surface diversity. Investigating this system may enhance our global understanding of microbial strategies for environmental adaptation, and may reveal common strategies used by several human and animal mycoplasma pathogens. the vlp genes from isogenic M. hyorhinis populations with defined variations in Vlp structure, size or expression, will be physically mapped, sequenced and expressed in order to determine genomic organization, structural and antigenic features, divergence and rearrangements mediating antigenic or phase variation. Restriction mapping, cloning, sequencing and expression of genomic fragments will be used. Accessible surface regions of Vlp antigens will be mapped with monoclonal antibodies on mutationally altered recombinant Vlp products, and transcription of vlp genes will be assessed in isogenic variants by standard methods.